When I go into Canon Photo Professional and change the dpi output to 300 or 350, do I also need to go down to resize setting and change those? The vinyl banner will be 48x108 with the photo being about 24x36. The shop tells me I need a photo of 300 dpi or higher but every picture I try to get info on from my computer says 72 dpi from my Canon Rebel XSI, yet my cheap Kodak point and shoot says 480 dpi. I am so confused. Can I take a picture with my 85mm lens, indoors, without flash and a bumped up ISO and get the 300 dpi they need for the banner? Or change the 72 dpi to 300? I know Canon Photo Professional will change the dpi but.... what will the blown-up pic look like?

4 Answers
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The DPI setting coming out of the camera doesn't mean much (if anything). You have a fixed number of pixels coming out of the camera (4272 x 2848, to be exact). Since you're enlarging it quite a bit, you probably want to shoot in raw format to ensure you get everything that camera can produce.

Once you've done that, you might want to "upres" the picture to fit what the printer has asked for. In this case, that'll be a resolution of 10800x7200 pixels, which means more than doubling the resolution in each dimension.

There are several ways to do that. There are some programs written specifically for that kind of task (e.g., Perfect Resize). You can also do this in Photoshop using `Image -> Image Size". There are tricks you might try using in Photoshop to see if they improve your results (but do look around -- different methods work better for different subject matter).

Once you're done, the picture shouldn't look drastically different on screen, though when/if you zoom in to 100% at the increased resolution, you can expect that it won't look particularly sharp -- a bit better than if you just zoom in to 250-300% on screen, but (honestly) not a whole lot. At the same time, it's a fair guess that people will generally be looking at a 48x108 banner from a fair distance away, not up nearly as close as most of us look at our monitors.

Given this level of enlargement, however, you do want to be as careful as possible to ensure the picture is sharp -- you probably do want to use either flash or a tripod (or both), especially shooting indoors (where the light is usually relatively dim). You probably also want to set your aperture to about 2 stops below the maximum for the lens to get the best performance it can provide.

If your subject is something that stays perfectly still, you could also consider shooting a number of overlapping pictures of parts of the subject, and stitching the pictures together. Photoshop can do this, and there are also tools specifically for the job such as Hugin and Microsoft ICE. These can give you a picture close to (or even greater than) the overall target resolution (but as I said above, this probably isn't a major problem).

Just as an aside here, there is one (count 'em, one) camera on the market right now that would give you what the printer wants straight out of the machine. It costs $50,000 and has been on the market for almost two months (!) now. Printers really need to get with the program -- they're not always going to be dealing with design professionals who know how to effectively up-rez an image, and they'd be in a better position to know when line-based (fractal, wavelet) or tone-based scaling is more appropriate for an image than the consumers they serve.
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user2719Apr 8 '11 at 1:48

@Stan: what camera is that? The highest resolution I knew of was the Leaf Aptus-II 12 (and 12R), which is close, but just short of this resolution.
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Jerry CoffinApr 8 '11 at 2:02

@Jerry: It's a Phase One/Leaf alright (same company with two different brands now). I believe the Leaf variant is called the Aptus-II 14 (and don't ask me what the 14 is supposed to mean) or it could be an Aptus-III these days; the one with Phase One badge is called IQ180 (that's Image Quality 1, 80 megapixel). Beyond gorgeous at ISO 50, and complete with an iOS-like touchy-swipy screen that will let you chimp shamelessly without ever actually taking a picture. Note: I may be willing to perform illegal, an am willing to perform immoral, acts in return for this camera.
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user2719Apr 8 '11 at 3:06

Ah, I'd heard of the IQ 180, but not checked the exact specs. Most of what I've heard was people debating whether to buy one, or pick up a used P65+, since there are almost certain to be quite a few coming on the market as others pick up IQ-180's. The new LCD does seem to be a big drawing point...
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Jerry CoffinApr 8 '11 at 3:14

Frankly, I'd be willing to "settle" on a P65+ myself. I do wish, though, that Phase One would work on better back/body integration (like 'Blad, but maybe without the matching). Not that it's a huge concern -- I have to save up my allowance for a lot of weeks before it becomes really important to me.
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user2719Apr 8 '11 at 3:20

Your confusion is understandable because the DPI in the photo is meaningless. It does not make any sense to speak about DPI without something that can be measured (like a print) but the cameras have to put something in there, so most put 72 and a few ones let you specify a different value.

To get the DPI of something divide the number of pixels by the size you want it printed on in each direction. Conversely multiply the size of the print (36"x24") by the requested DPI (300) to get the resolution you need (10800 x 7200) pixels in your case.

This short article explains this in details and has a calculator where you can input two of DPI, resolution and size and it calculates the missing one.

The value of 72 is actually mandated by the EXIF standard. Who lets you change it?
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Mark RansomApr 8 '11 at 2:13

@Mark - I did not know that! Why? The Olympus E-PL2 for example lets you change it between 1 and 9999 DPI or Auto. I've seen it in other cameras too, but don't remember which ones. It never had any importance to me.
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ItaiApr 8 '11 at 3:11

@Mark - As the OP mentioned, he has one camera that puts 480 DPI, I've seen other numbers (In the 72-360 range) with 72 being the most common.
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ItaiApr 8 '11 at 3:14

1

The resolution field is intended for devices which have an actual physical resolution, such as scanners. The EXIF 2.2 spec is at exif.org/Exif2-2.PDF, have a look at page 19 (which is page 25 of the PDF).
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Mark RansomApr 8 '11 at 3:41

That doesn't appear to be a mandate, just a default.
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mattdmApr 8 '11 at 10:28

Shoot RAW instead of JPEG, that shall give you 300 dpi in EXIF after converting.

Now about capturing photos which you know will be enlarged for printing as much as twice/thrice their original size, no matter what tool you use, you can not generate pixels out of the blue which your camera didnt take. So, why not take several photos and stitch them to get the full size in one photo while keeping the IQ the very same? You can try using Brenizer Method. You already have a 85mm, and you're ready to go. Please check this link out for details on how to do it:
http://blog.buiphotography.com/2009/07/the-brenizer-method-explained-with-directions/

Its a fairly simple method and doesn't hurt much. I have been trying this for last few days and I love it =)

Your camera will record a certain number of pixels, which is a fixed number based on the sensor. Let's say it's 3000 pixels by 4000 pixels = 12 megapixels.

A computer screen typically has a standard of 72 dpi (dots per inch), so if you want to show the image at 100% on the screen, at 72 pixels per inch it will be 4000 divided by 72 = 59 inches wide.

Printers can place the dots/pixels much closer together, and if you don't do this, you can see "pixelation". So at 300 dpi, that's only 4000 / 300 = 13 inches.

With an image editor like photoshop you can increase the image size by interpolating pixels, and you can preview what it might look like when printed (or print off a small section as a test). As long as you're not viewing too close it usually looks fine in my experience. It depends on the subject matter and the viewing distance to the photo/banner.